Patent GB-992730-A (GB)
Title: Improvements in control systems for a.c. motors
Inventor: BOWLER PETER
IPC: H02P27/02
Filed: 1962-09-17
Abstract: 992,730. Automatic speed control. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Sept. 16, 1963 [Sept. 17,1962], No. 35320/62. Heading G3R. [Also in Division H2] A squirrel cage induction motor is supplied at variable frequency by a waveform copying device receiving three-pulse signals from a unit of the type described in Specification 952, 945 which determines the phase sequence and frequency of the signals. A control voltage for the unit is provided by a slip-limiting circuit to which is applied a reference voltage at terminal 13 and the output of a motor-driven tachometer generator over line 15. As long as the difference between the emitter voltages of transistors T1, T2 is small, corresponding to a difference between the desired and actual speeds of less than a predetermined amount, the signal applied to the unit is not modified by the action of the tachometer generator. However, if the error voltage increases, diodes 25 become effective to limit the output signal on terminal 19 so that the desired change in speed is effected at a rate which avoids excessive slip between the rotor and the rotating field. The tachometer generator may be a D.C. machine, or an A.C. machine with an inductive pick-off. The width of the slip-range can be changed by the setting of potentiometer 21, and diodes 25 may be replaced by Zener diodes. When a variable slip-range is required, alternative output terminal 23 is used. The transistors T1, T2, act as buffer amplifiers of unity gain. The waveform copying device comprises silicon controlled rectifiers arranged in six groups of three, each group being connected at one end to a three phase supply and their opposite terminals joined to a centre-tapped reactor feeding the output leads, Fig. 3 (not shown). The load voltage varies at the same frequency as the firing delay variations, and by appropriate timing of the firing delay, individual phase contributions are of corect magnitude to provide a sinusoidal output at the desired frequency plus harmonics of the supply. In another arrangement, for synchronous motors, the slip limiting unit is replaced by a unit limiting the rotor phase angle, the tachometer generator being replaced by a unit sensitive to the actual rotor phase angle. The synchronous motor 101 is coupled to generator 103 and is energized from waveform copying device 105 receiving input signals over leads 107 from unit 108. The phase sequence and frequency of the signals in leads 107 are determined by the control voltage on leads 109 associated with rotor phase angle limiter 111 supplied with a reference voltage at terminal 113. The signals in leads 107 are also supplied to modulator 115 together with a carrier signal from lead 116, the output of the modulator being applied to the stator of generator 103 which provides a signal phase output signal. The output in leads 119 is determined by the load angle of the motor 101, the leads 119 and leads 107 being connected to a phase conscious rectifier 123 acting as a phase comparator whose output is applied to rotor phase angle limiting unit 111. The unit 111 limits the frequency of the motor current to a value corresponding to the speed and prevents the motor from falling out of synchronism as it accelerates or decelerates, despite the setting of the desired motor speed by the selection of the voltage at terminal 113. The system also provides control during regenerative braking operation.